In image forming methods as typified by an electrophotographic recording method, a developer charged electrostatically (hereinafter “toner”) flies to the surface of a photosensitive member by electrostatic force which accords with potential differences on the photosensitive member surface, and develops electrostatic latent images formed on the photosensitive member surface. Hence, it is necessary and indispensable to control charge characteristics of the toner. Then, as a method for providing the toner with proper charge characteristics, a method is known in which a binder resin of a developer bearing member (hereinafter also “developing roller”) is incorporated with a charge control agent or a method in which a charge control agent capable of providing the toner with positive charges or negative charges is mixed into toner particles to control the chargeability of the toner.
Conventionally, as negatively charging charge control agents, metal complexes of monoazo dyes, metal complexes of salicylic acid, alkylsalicylic acids or benzilic acid and so forth are used (PTL 1).
In PTL 2, it is proposed that a binder resin of a developing roller is incorporated with such a negatively charging charge control agent and this can provide a positively chargeable developer with a sufficient charge quantity by triboelectric charging.
Recently, because of safety, concern about environments and a requirement for stabler charge characteristics, it is proposed to use as the charge control agent a resin having charge control function. In PTL 3, it is proposed that a resin having a sulfonic acid group is mixed in a binder resin of the developing roller to thereby improve the developer in its uniformity of triboelectric charging and its running stability.
It is also disclosed as reported in PTL 4 that, as a polymeric charge control agent, a polymeric compound composed of a monomer unit having a sulfonic acid group is mixed into toner particles to thereby improve the charging stability of the toner and the compatibility of the compound itself with a binder resin.
Meanwhile, in recent years, images to be reproduced are desired to be of much higher image quality, and faulty images have come into question, such that “fog” occurs where the toner comes into development at blank or white areas of images and that any streaky density non-uniformity occurs on images. What is a large factor of such a phenomenon is the occurrence of a toner having come charged to a polarity reverse to the desired charge polarity.
For the purpose of making a toner charged to a reverse polarity less form or keeping the toner from being so charged, PTL 5 discloses therein an example in which a salicylic acid metal complex is used as a charge control agent, and PTL 6 discloses therein an example in which a polymeric compound composed of a monomer unit having a sulfonic acid group is used as a charge control agent. PTL 7 further discloses therein an example in which a polymeric compound to the polymeric backbone chain of which a salicylic acid unit is directly linked is used in the toner.